Graphene-semiconductor based wavelength selective photodetector for sub-bandgap photo detection

ABSTRACT

Graphene photodetectors capable of operating in the sub-bandgap region relative to the bandgap of semiconductor nanoparticles, as well as methods of manufacturing the same, are provided. A photodetector can include a layer of graphene, a layer of semiconductor nanoparticles, a dielectric layer, a supporting medium, and a packaging layer. The semiconductor nanoparticles can be semiconductors with bandgaps larger than the energy of photons meant to be detected.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application Ser.No. 62/346,706, filed Jun. 7, 2016, the disclosure of which is herebyincorporated by reference in its entirety including any tables, figures,or drawings.

FIELD OF INVENTION

The subject invention relates to a photodetector with the capability tooperate in the sub-bandgap region relative to the bandgap ofsemiconductor nanoparticles. More specifically, the subject inventionrelates to the use of a graphene-semiconductor hybrid structure forphoto detection in the wavelength beyond the wavelength corresponding tothe semiconductor bandgap.

BACKGROUND OF THE INVENTION

Due to its superior electrical and optical properties, such as highcarrier mobility and gapless band structure, graphene has beenconsidered as a promising material to replace existing photoactivesemiconductor materials for high speed, high sensitivity and broadbandphoto detection. See Xia F N, Mueller T, Lin Y M, Valdes-Garcia A,Avouris P., Ultrafast graphene photodetector. Nat Nanotechnol 4, 839-843(2009) (hereinafter referred to as “Xia”) and Nair R R, et al., Finestructure constant defines visual transparency of graphene. Science 320,1308-1308 (2008) (hereinafter referred to as “Nair”), both of which areincorporated herein in their entirety. The photo response in agraphene-based photodetector is mainly attributed to three mechanisms:(i) the photovoltaic effect, (ii) the photothermoelectric effect, and(iii) the photogating effect. See Lemme M C, et al., Gate-ActivatedPhotoresponse in a Graphene p-n Junction. Nano Lett 11, 4134-4137 (2011)(hereinafter referred to as “Lemme”); Mueller T, Xia F N A, Avouris P.Graphene photodetectors for high-speed optical communications. NatPhotonics 4, 297-301 (2010) (“Mueller”); Gabor N M, et al., HotCarrier-Assisted Intrinsic Photoresponse in Graphene. Science 334,648-652 (2011) (hereinafter referred to as “Gabor”); Sun D, et al.,Ultrafast hot-carrier-dominated photocurrent in graphene. NatNanotechnol 7, 114-118 (2012) (hereinafter referred to as “Sun”);Tielrooij K J, et al., Generation of photovoltage in graphene on afemtosecond timescale through efficient carrier heating. Nat Nano 10,437-443 (2015) (hereinafter referred to as “Tielrooij”); Konstantatos G,et al., Hybrid graphene-quantum dot phototransistors with ultrahighgain. Nat Nanotechnol 7, 363-368 (2012) (hereinafter referred to as“Konstantatos”); and Zhang D Y, Gan L, Cao Y, Wang Q, Qi L M, Guo X F.,Understanding Charge Transfer at PbS-Decorated Graphene Surfaces towarda Tunable Photosensor (hereinafter referred to as “Zhang”), all of whichare incorporated herein in their entirety.

In addition to enhanced sensitivity, wavelength selectivity is adesirable characteristic for photodetector in certain applications. Thespectral selectivity is determined by integrating microcavity, waveguideor metal plasmonic structures. See, Furchi M, et al.,Microcavity-Integrated Graphene Photodetector. Nano Lett 12, 2773-2777(2012) (hereinafter referred to as “Furchi”); Gan X T, et al.,Chip-integrated ultrafast graphene photodetector with high responsivity.Nat Photonics 7, 883-887 (2013) (hereinafter referred to as “Gan”); andEchtermeyer T J, et al., Strong plasmonic enhancement of photovoltage ingraphene. Nat Commun 2, (2011) (hereinafter referred to as“Echtemeyer”), all of which are incorporated herein in their entirety.Although, as in metal plasmonic structures, the intrinsic plasmonicabsorption in graphene nanostructures is primarily determined by theirgeometry, the low density of states promises graphene the potential oftuning the light absorption by electrostatic gating. See, Freitag M, LowT, Zhu W J, Yan H G, Xia F N, Avouris P, Photocurrent in grapheneharnessed by tunable intrinsic plasmons. Nat Commun 4, (2013)(hereinafter referred to as “Freitag”) and Chen J N, et al., Opticalnano-imaging of gate-tunable graphene plasmons. Nature 487, 77-81 (2012)(hereinafter referred to as “Chen 2”), both of which are incorporatedherein in their entirety. However, in addition to the high costassociated with a nanofabrication process, all of these photodetectorsrely on the intrinsic photo response of graphene, leading to a lowresponsivity of <1 A·W⁻¹, which limits its potential applications. It iswell known that the silicon oxide layer on silicon wafer can serve as aFabry-Perot cavity, which is responsible for the optical visibility ofsingle layer graphene. Abergel D S L, Russell A, Fal'ko V I, Visibilityof graphene flakes on a dielectric substrate. Applied Physics Letters91, 063125 (2007) (hereinafter referred to as “Abergel”) and Roddaro S,Pingue P, Piazza V, Pellegrini V, Beltram F, The Optical Visibility ofGraphene: Interference Colors of Ultrathin Graphite on SiO2. Nano Lett7, 2707-2710 (2007) hereinafter referred to as (“Roddaro”), both ofwhich are incorporated herein in their entirety.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the subject invention provide devices and fabricationmethods for a graphene photodetector that absorbs photons in a graphenelayer in contrast to traditional semiconductor photodetectors. Thephotodetector of the subject invention is inspired by traditional silverhalide photographic film, which enables a photo response to photons withsub-bandgap energy. Upon illumination, the graphene layer absorbsphotons and hot electrons are generated in the graphene. Thesephoton-excited hot electrons can transfer to the conduction band of thesilver halide nanoparticles. The hot electrons promote a redox reactionin the silver halide, reducing Ag⁺ to Ag⁰. The Ag⁻ serves as a chemicalelectron reservoir, which enhances the lifetime of trapped chargecarriers and results in high responsivity of the photodetector. Due tothe low absorbance of the graphene layer, the design enables a noveltransparent visible light photodetector.

In an embodiment, the photodetector comprises a graphene channel, alayer of semiconductor nanoparticles, a supporting medium, and apackaging layer. The semiconductor nanoparticles act as the reservoir tostore the photo carriers excited from the graphene and as a photogatefor the graphene channel, leading to the change of conductance in thegraphene layer. In an embodiment, this sub-bandgap absorption can alsobe realized by combining graphene with other form of semiconductors,such as semiconductor films.

Using the principles in Lien D H, et al., Engineering Light Outcouplingin 2D Materials. Nano Lett 15, 1356-1361 (2015) (hereinafter referred toas “Lien”), which is incorporated herein by reference in its entirety,an embodiment of the subject invention uses simple wavelength-selectiveenhancement techniques on the photodetector in order to provide largearea modulation without complicated sub-micron fabrication techniques.

The semiconductor nanoparticles in an embodiment of the subjectinvention can be chemically modified or dye sensitized to adjust theabsorption properties. The position of the semiconductor nanoparticlesis not limited to the upper surface of the graphene. They can be coveredby the graphene or mixed with graphene flakes from liquid exfoliation.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects and advantages of the subject inventionwill become more apparent when considered in connection with thefollowing detailed description and appended drawings in which likedesignations denote like elements in the various views, and wherein:

FIGS. 1(a)-1(h) show images of a possible process flow for fabricating aphotodetector device according to an embodiment of the subjectinvention, in which FIG. 1(a) shows CVD grown graphene on a substrate(Si or quartz) patterned into ribbons using e-beam lithography andoxygen RIE, FIG. 1(b) illustrates 0.5 nm Ag deposited on the centralarea of the graphene ribbon by thermal deposition, FIG. 1(c) shows theremoval of PMMA by a mixture of dichloroethene and acetone, FIG. 1(d)shows the transformation of the Ag into AgCl by a reaction with Cl₂,FIG. 1(e) illustrates the coating of the sample with 5 nm Al₂O₃, FIG.1(f) shows that after removing the Al₂O₃ on the contact area the deviseis dipped into a buffered HF solution (BHF) and nickel (Ni) is sputteredon as the electrodes, FIG. 1(g) shows cleaning of the PMMA by a mixtureof dichloroethene and acetone and FIG. 1(h) is a top view of thefinished photodetector.

FIGS. 2(a)-2(f) are an overview of the device according to the subjectinvention and the concepts upon which it is designed, in which FIG. 2(a)shows an image of a schematic of a AgCl/graphene photodetector accordingto an embodiment of the subject invention, FIG. 2(b) is an image of anexploded view of a graphene photodetector according to an embodiment ofthe subject invention, FIG. 2(c) shows a schematic of a photocarriergeneration mechanism, FIG. 2(d) shows an image of a plot of temporalphoto response of a device according to the an embodiment of subjectinvention, FIG. 2(e) shows an image of a plot of a photocurrent responseof a device according to an embodiment of the subject invention after 6months storage in ambient conditions and FIG. 2(f) shows a plot of thephotocurrent dependence on back-gate voltage from −200 V to 200 V for anembodiment of the subject invention.

FIGS. 3(a)-3(c) show a photo response of AgCl/graphene device accordingto an embodiment of the subject invention on transparent substrates, inwhich FIG. 3(a) shows a plot of the photon-to-current efficiency of aAgCl/graphene device on quartz as a function of excitation wavelength at1V bias, FIG. 3(b) shows an image of a flexible AgCl/graphenephotodetector on PET film, and FIG. 3(c) shows an image of a buildingthrough flexible AgCl/graphene photodetector on PET film.

FIGS. 4(a)-4(c) are a wavelength-resolved photo response, in which FIG.4(a) shows a plot of the responsivity of a AgCl/graphene device on 470nm SiO₂ as a function of excitation wavelength, FIG. 4(b) shows a plotof the responsivity of the AgCl/graphene device on 470 nm SiO₂ as afunction of source-drain bias (V_(sd)) measured under varyingilluminating power at 400 nm wavelength, and FIG. 4(c) shows a plot ofthe responsivity of the AgCl/graphene device on 470 nm SiO₂ as afunction of bias V_(sd) measured under varying illuminating power at 500nm wavelength.

FIGS. 5(a)-5(c) are a FDTD simulation for a wavelength selectiveenhancement, in which FIG. 5(a) shows a plot of a normalized intensitymap of an absorption cross section for AgCl/graphene as a function ofboth the excitation wavelength and the SiO₂ thickness, FIG. 5(b) shows agraph comparing the absorption cross section spectra of AgCl/graphenewith pure graphene on a 470 nm thick SiO₂ substrate, and FIG. 5(c) showsa plot of the power absorption per unit area plot for an AgCl/graphenedevice on 470 nm SiO₂ at 400 nm wavelength.

FIG. 6 shows a plot of the responsivity versus wavelength forAgL/graphene and AgBr/graphene.

FIGS. 7(a)-7(f) show an AgCl-graphene photodetector and temporalphotoresponse, in which FIG. 7(a) shows the schematic diagram of thesilver halide-graphene photodetector, FIG. 7(b) shows a temporalphotoresponse of the as-prepared AgX-G hybrid photodetectors, FIG. 7(c)shows an energy band diagram at the junction formed by graphene andsemiconducting AgX, FIG. 7(d) shows a long time (1000 cycles) temporalphotoresponse of AgCl-G photodetector, after six months preservationunder ambient conditions, FIG. 7(e) shows a schematic diagram of thephoto-electro-chemical measurement setup for internal photoemission atAgCl-graphene junction, and FIG. 7(f) shows a temporalphotoelectrochemical response of AgCl-G working electrode at 0 V biasunder chopped green light illumination (520 nm wavelength).

FIGS. 8(a)-8(d) show the spectral selectivity modulation characteristicsof an embodiment of the subject invention, in which FIG. 8(a) shows anillustration of the multiple reflections within the dielectric layerbeneath the monolayer graphene, FIG. 8(b) shows an experimentalwavelength-dependent responsivity of AgCl-G on thermal oxide substrates,FIG. 8(c) shows an absorption intensity map for the AgCl-G as a functionof both the excitation wavelength and the SiO₂ thickness, and FIG. 8(d)shows a calculated absorption of AgCl-G and bare graphene on 470 nmSiO₂.

DETAILED DISCLOSURE OF THE INVENTION

Embodiments of the subject invention provide novel and advantageousgraphene photodetectors and fabrication methods thereof. In certainembodiments, photovoltaic electron-hole pairs generated in the graphenelayer can be separated by the Schottky barrier at thegraphene-semiconductor interphase. The photo generated electrons can beinjected into the semiconductor nanoparticles and the holes left in thegraphene channel alter the conductance of the graphene channel so as togenerate a response signal. The semiconductor nanoparticles can act asreservoirs for the photo generated electrons.

Due to its gapless band structure, the photo-generated exciton ingraphene recombines in the sub-picosecond time scale (see, for example,Xia). As a result, it is necessary to employ a built-in electric fieldto dissociate electron-hole pairs in order to show pronouncedphotovoltaic effect. This electric field is usually established bychemical doping, electrostatic gating or contact with metal of adifferent work function. See Kim C O, et al., High photoresponsivity inan all-graphene p-n vertical junction photodetector. Nat Commun 5,(2014) (hereinafter referred to as “CO”), which is incorporated hereinin its entirety.

Accompanying the photovoltaic effect, a photo-thermo-electric effectwill also occur when light is absorbed around the p-n junction ofgraphene. Unfortunately, due to its small photoactive area orinsignificant Seebeck coefficient difference, together with the weaklight absorption (˜2.3%) of intrinsic graphene, the overall responsivityof related art graphene photodetectors are less than 100 mA·W⁻¹ (see,for example, Xia and Nair). In contrast, ultrahigh photoconductive gaincan be obtained by the photogating effect in graphene hybridphototransistors. See Konstantatos, Zhang and Klekachev A V, Cantoro M,van der Veen M H, Stesmans A L, Heyns M M, De Gendt S., Electronaccumulation in graphene by interaction with optically excited quantumdots. Physica E: Low-dimensional Systems and Nanostructures 43,1046-1049 (2011) (hereinafter referred to as “Klekachev”), which isincorporated herein in its entirety.

In the case of photogating, semiconductor quantum dots or organic dyemolecules serve as a light absorbing layer for photo-carrier generation.Upon illumination, photon excited charge carriers are injected into thegraphene underneath and the rest of the oppositely charged carrierstrapped in the quantum dots or dye layer can modulate the conductance ofgraphene by capacitive coupling. Due to the long lifetime of trappedcarriers (τ_(lifetime)) in the quantum dot layer, extraordinaryresponsivity, up to 10⁷ A·W⁻¹, has been realized in this hybridstructure. As described in this proposed mechanism, the photo responseof the hybrid transistor originates from photoexcitation of theincorporated photoactive absorber other than the graphene itself. Itseems like the spectral response of the hybrid photodetector isfundamentally limited by the absorption of incorporated quantum dots ordyes as shown in PbS-graphene, ZnO-graphene, perovskite-graphene andchlorophyll-graphene hybrids. See Konstantatos, Shao D, et al.,Organic-Inorganic Heterointerfaces for Ultrasensitive Detection ofUltraviolet Light. Nano Lett 15, 3787-3792 (2015) (hereinafter referredto as “Shao”); Dang V Q, et al., Ultrahigh Responsivity in Graphene-ZnONanorod Hybrid UV Photodetector. Small 11, 3054-3065 (2015) (“Dang”);Lee Y, et al., High-Performance Perovskite-Graphene HybridPhotodetector. Adv Mater 27, 41-46 (2015) (hereinafter referred to as“Lee”); and Chen S-Y, et al., Biologically inspired graphene-chlorophyllphototransistors with high gain. Carbon 63, 23-29 (2013) (hereinafterreferred to as “Chen”), all of which are incorporated herein in theirentirety.

In order to fully benefit from the broad absorption spectrum ofgraphene, a double-layer graphene device can use two layers of graphenethat are isolated by a thin dielectric barrier. See, Liu C H, Chang Y C,Norris T B, Zhong Z H, Graphene photodetectors with ultra-broadband andhigh responsivity at room temperature. Nat Nanotechnol 9, 273-278 (2014)(hereinafter referred to as “Liu”), which is incorporated herein in itsentirety. The trapped charges in the top graphene layer can impose aphotogating effect on the bottom graphene layer, bringing in a highresponsivity of ˜1000 A·W⁻¹ at 532 nm and ˜1 A·W⁻¹ in the mid-infraredrange. However, the complicated device structure and requirement ofusing high reset back-gate voltage pulses synchronized with the externallight input signal may limit its potential application.

Embodiments of the subject invention overcome the problems of therelated art and enable sub-bandgap photo detection with enhancedresponsivity compared to pure semiconductor-based photodetectors.According to embodiments of the subject invention, a dielectric layercan be modulated to realize large scale wavelength selective enhancementand function as a key component in flexible and transparentoptoelectronics for imaging, spectroscopy, sensing and opticalcommunications.

In an embodiment of the subject invention, a photodetector can comprisea layer of graphene, a layer of semiconductor nanoparticles, asupporting medium, and a packaging layer. Novel features include (a)sub-bandgap absorption, (b) wavelength selective enhancement, (c)flexibility, and (d) transparency. In an embodiment, the photodetectorcan be in the form of AgX/graphene, where X═Cl, Br, or I. For example,the silver halide can be AgCl. The supporting medium can comprise one ormore of the following: quartz; thermal oxidized Si; sapphire; siliconcarbide; aluminum nitride; polydimethylsiloxane; and flexible plasticsubstrates (such as polyester, polyimide, polyethylene naphthalate,polyetherimide, and fluoropolymers et al.).

Photodetector Fabrication.

As shown in FIGS. 1(a)-1(h), in an embodiment of the subject invention,a monolayer chemical vapor deposition (“CVD”) graphene on copper isspin-coated (4,000 rpm for 1 minute) with 7 wt. % Poly(methylmethacrylate) (PMMA) solution in anisole and dried in air. The grapheneon the reverse side of copper is removed through oxygen reactive ionetching (RIE) for approximately 2 min. The copper is then removed bychemically etching the copper in 0.1 M (NH₄)₂S₂O₈ aqueous solution. Thereleased graphene, which is now attached on the PMMA film, ismechanically removed and rinsed consecutively in several clean deionized(“DI”) water baths. Finally, the film is applied to a clean siliconsubstrate with 470 nm or 200 nm thermal oxide, quartz or polyethyleneterephthalate (PET) film and dried in air. After dissolving the PMMAwith acetone, the graphene is patterned into a ribbon with e-beamlithography and oxygen, as seen in FIG. 1(a). 0.5 nm Silver (Ag)nanoparticles are deposited onto the graphene by thermal evaporation.Electronic beam lithography is used to pattern a central area of thegraphene into micro-ribbons, as seen in FIG. 1(b). FIG. 1(c) shows theremoval of PMMA by a mixture of dichloroethene and acetone.

In an embodiment, gas-solid reactions are used to synthesize the Agnanoparticles into silver chloride (AgCl) nanoparticles. In particular,FIG. 1(d) shows the transformation of the Ag into AgCl by a reactionwith chlorine (Cl₂). A 5 mL glass vial with the sample substrate iscovered by a layer of silica gel and placed in a sealed 25 mL glass vialwith 0.05 g solid potassium chlorate (KClO₃.) Then 0.15 mL of 37%hydrochloric acid (HCl) solution is added to the solid and the Cl₂ isgenerated in-situ (KClO₃+6 HCl=KCl+5 Cl₂+3 H₂O) and allowed to reactwith the evaporated Ag nanoparticles for 10 minutes at room temperature.

In an embodiment, in order to synthesize silver bromide (AgBr) tofabricate AgBr/graphene, the following steps were taken. A 5 mL glassvial with a sample substrate is covered by a layer of silica gel andplaced in a sealed 25 mL glass vial with 0.05 g solid potassium bromide(KBr). Then 0.15 mL of solid potassium permanganate (KMnO₄) and 98%sulfuric acid (H₂SO₄) mixture solution is added to the solid and Br₂vapor is produced by the reaction (2 KMnO₄+8 H₂SO₄+10 KBr=6 K₂SO₄+5Br₂+2 MnSO₄+8 H₂O) and allowed to react with Ag nanoparticle forapproximately 10 minutes at room temperature.

In an embodiment, in order to synthesize silver iodide (AgI) for aAgI/graphene photodetector, the sample substrate is placed in a sealed30 mL glass vial containing 0.05 g of solid iodine (I₂). Then thereaction vial is heated to 105° C. to allow the I₂ vapor to react withAg nanoparticles for 10 minutes. After the reaction, 5 nm of Al₂O₃ iscoated by atomic layer deposition (ALD) using trimethylaluminum as aprecursor at 70° C. The Al₂O₃ minimizes the influence of ambient gasmolecule adsorption and desorption upon illumination.

FIG. 1(e) illustrates the coating of the sample with 5 nm aluminum oxide(Al₂O₃) in an embodiment of the subject invention. The Al₂O₃ on thecontact area is removed by dipping it in 5:1 buffered HF solution (BHF)for 1 second, and a 100 nm nickel (Ni) electrode is patterned on thegraphene ribbon with e-beam lithography followed by metal sputtering. Inparticular, FIG. 1(f) shows that after removing the Al₂O₃ on the contactarea the device is dipped it into a buffered HF solution (BHF) andnickel (Ni) is sputtered on as the electrodes. FIG. 1(g) shows cleaningof the PMMA by a mixture of dichloroethene and acetone and FIG. 1(h) isan optical image of the finished photodetector.

The Mechanism of the Graphene Photodetector

For a graphene photodetector, many semiconductor materials with variousband gap energy levels, such as ZnO (E_(g)˜3.3 eV), organolead halideperovskite (CH₃NH₃PbX₃, E_(g)˜1.5 eV) and PbS (E_(g)˜0.37 eV), have beenhybridized with graphene and shown to exhibit extraordinary sensitivityenhancement over pristine graphene (see, for example, Lee and Wang Y, etal. Hybrid Graphene-Perovskite Phototransistors with UltrahighResponsivity and Gain. Advanced Optical Materials 3, 1389-1396 (2015)(hereinafter referred to as “Wang”), which is incorporated herein in itsentirety. In those designs, since the semiconductor is responsible forthe photon absorption, the responsivity spectrum of the photodetectoraligns with the absorption spectrum of the incorporated semiconductor.In an embodiment of the subject invention, this limitation can beovercome with a silver halide-graphene hybrid system, wherein thegraphene itself serves as the photosensitizer by injecting excited hotcarrier electrons into the conduction band of a silver halide and usinga reversible Ag⁺/Ag⁰ redox couple as a reservoir for said hot carrierelectrons. This innovation enables a strong photo response for longwavelength photons beyond the absorption edge of an incorporatedsemiconductor.

A schematic of a silver halide-graphene hybrid photodetector is shown inFIG. 2(a) and FIG. 2(b) in an embodiment of the subject invention. Thesingle layer chemical vapor deposition (CVD) grown graphene is incontact with a layer of well-dispersed silver chloride (AgCl)nanoparticles. Due to its work function (Φ_(AgCl)=4.8 eV) and bandstructure (E_(g)=3.25 eV, E_(C)=4.3 eV and E_(V)=7.55 eV), a Schottkybarrier is formed at the AgCl-graphene interface. See, Seiichi Sumi T W,Akira Fujishima, Kenichi Honda, Effect of Cl- and Br-Ions and pH on theFlatband Potentials of Silver Halide Sheet Crystal Electrodes. B ChemSoc Jpn 53, 2742-2747 (hereinafter referred to as “Sumi”); Bose D N,Govindacharyulu P A., Physics of silver halides and their applications.Bull Mater Sci 2, 221-231 (1980) (hereinafter referred to as “Bose”);and Bauer R S, Spicer W E, Silver-halide valence and conduction states:Temperature-dependent ultraviolet-photoemission studies. Phys Rev B 14,4539-4550 (1976) (hereinafter referred to as “Bauer”), all of which areincorporated herein in their entirety.

The as-prepared device with a p-type doped (ΔE_(F) shift of fermi levelfrom the Dirac point Φ_(g) ⁰) graphene generates electron-hole pairsupon photoexcitation, according to an embodiment of the subjectinvention. The photo-generated electrons are injected through theSchottky barrier into AgCl nanoparticles and stored in Ag⁺, reducing itto Ag. The holes left in the graphene channel increase the ΔE_(F) andshift the graphene fermi level to a more p-type position, changing theconductance of the graphene channel through the photogating effect. Thevisible light illumination cannot generate photocarriers from AgClparticles due to its large band gap (3.25 eV), highlighting thesub-bandgap absorption in the graphene-AgCl heterostructure.

As a photon is absorbed by graphene, hot electron-hole pairs aregenerated and separated by the built-in field in the Schottky barrier atthe graphene-semiconductor interface (see, for example, FIG. 2(c)),largely preserving the merits of gapless band structure and gatetunability of pristine graphene. See, Knight M W, Sobhani H, NordlanderP, Halas N J, Photodetection with Active Optical Antennas. Science 332,702-704 (2011) (hereinafter referred to as “Knight”); and Zheng B Y,Zhao H, Manjavacas A, McClain M, Nordlander P, Halas N J, Distinguishingbetween plasmon-induced and photoexcited carriers in a device geometry.Nat Commun 6, (2015) (hereinafter referred to as “Zheng”), which areincorporated herein in their entirety. Due to the gapless band structureof graphene and the ultrafast intraband recombination of photo-generatedelectrons, low hot carrier injection efficiency is expected afterexcitation. See, Breusing M, Ropers C, Elsaesser T, Ultrafast CarrierDynamics in Graphite. Phys Rev Lett 102, 086809 (2009) (hereinafterreferred to as “Breusing”), which is incorporated herein in itsentirety. Nevertheless, a small portion of hot electrons with sufficientenergy can be injected into the conduction band of AgCl and promote thereduction of Ag⁺ to Ag⁰ with E(Ag⁺/Ag⁰)=4.66 eV. Vanýsek P, CRC Handbookof Chemistry and Physics, 96th ed. CRC Press (2015) (hereinafterreferred to as “Vanýsek”), which is incorporated herein in its entirety.This negatively charged AgCl layer then modulates the conductance of thegraphene channel through the photogating effect. In this mechanism, theAg⁺/Ag⁰ redox couple serves as a reversible chemical electron reservoirand electrons are stored in Ag nanoparticles, which are spatiallyseparated from the graphene layer. Compared with previously reportedlead sulfide (PbS) and zinc oxide (ZnO) hybrid systems, the existence ofspatially separated chemical electron reservoirs significantlystabilizes the injected electrons and promotes the sensitivity of lowenergy photons beyond the band gap limit of the semiconductor. Suchresults not only offer a new paradigm of sub-bandgap absorption in a lowSchottky barrier graphene-semiconductor heterostructure, but they alsopave the way for developing chemically adjustable optoelectronic systemsbased on other 2D layered materials. This hybrid graphene-based deviceis envisaged to be important for flexible and transparentoptoelectronics for imaging, spectroscopy, sensing and opticalcommunications.

According to an embodiment of the subject invention, the temporalresponse to white light of the AgCl/graphene photodetector is shown inthe FIG. 2(d) for a device prepared with AgX/graphene (470 nm SiO₂)photodetectors (X═Cl, Br, I) with the same ˜54.9 μW white light over the22×16 μm² graphene channel area. The photocurrent of AgI/graphene hasbeen increased 5 times for clarity. The photocurrent is recorded bysubtracting the dark current from the light current, which graduallydecreases from AgCl to AgBr and AgI, under the same illumination power˜54.9 μW from a white LED on the graphene channel area (channel lengthof 22 μm and width of 16 μm). This decrease of responsivity from AgCl toAgI could be attributed to the increased conduction band edge andsubsequently higher Schottky barrier or the lower reduction efficiencyof Ag⁺/Ag⁰ caused by the smaller drift mobility of the photoelectrons inAgBr and AgI. See, Dong H, et al., Highly-effective photocatalyticproperties and interfacial transfer efficiencies of charge carriers forthe novel Ag₂CO₃/AgX heterojunctions achieved by surface modification,Dalton Trans 43, 7282-7289 (2014) (hereinafter referred to as “Dong”)and Tani T, Explanation of Photocatalytic Water Splitting by SilverChloride from Viewpoint of Solid State Physics and PhotographicSensitivity of Silver Halides, Journal of The Society of PhotographicScience and Technology of Japan 72, 88-94 (2009) (hereinafter referredto as “Tani”), which are incorporated herein in their entirety.

The subject invention includes, but is not limited to, the followingexemplified embodiments.

Embodiment 1

A photodetector comprising:

a supporting medium;

a layer of graphene;

a layer of semiconductor nanoparticles; and

a packaging layer.

Embodiment 2

The photodetector of embodiment 1, wherein the semiconductornanoparticles are on the graphene layer.

Embodiment 3

The photodetector of any of embodiments 1-2, wherein the semiconductornanoparticles are covered by the graphene layer.

Embodiment 4

The photodetector of any of embodiments 1-3, wherein the semiconductornanoparticles are mixed with graphene flakes.

Embodiment 5

The photodetector of any of embodiments 1-4, wherein the diameter ofsemiconductor nanoparticles can vary from the nanometer range to themicrometer range.

Embodiment 6

The photodetector of any of embodiments 1-5, wherein the semiconductornanoparticles are semiconductors with bandgaps larger than the aimedenergy of photons.

Embodiment 7

The photodetector of any of embodiments 1-6, wherein silver halidenanoparticles are used for detecting photons in a sub-bandgap region.

Embodiment 8

The photodetector of any of embodiments 1-7, wherein the semiconductornanoparticles are sensitized by organic or inorganic dyes so as tomodulate their properties.

Embodiment 9

The photodetector of any of embodiments 1-8, wherein the supportingmedium is integrated with a photonic structure, such as a micro cavity,waveguide, or metal plasmonic structure, to modulate the performance ofthe device.

Embodiment 10

The photodetector of any of embodiments 1-9, wherein the packaging layeris an organic or inorganic layer with sufficient transmittance in theaimed range of detection.

Embodiment 11

The photodetector of any of embodiments 1-10, wherein the thickness ofthe supporting medium or packaging layer is modulated to achieveselective enhancement of photo detection.

Embodiment 12

The photodetector of any of embodiments 1-11, which is used in at leastone of the following: flexible and transparent optoelectronics forimaging, spectroscopy, sensing, and optical communications.

Embodiment 13

A method of manufacturing a graphene semiconductor photodetector, themethod comprising:

providing a monolayer chemical vapor deposition (“CVD”) graphene on ametal (e.g., copper);

spin-coating a poly(methyl methacrylate) (PMMA) solution in anisole ontothe graphene layer and air drying it;

removing the metal (e.g., copper) on the reverse side of the graphene;

scooping out the released graphene on the PMMA film and rinsing the filmin at least one deionized (“DI”) water bath (e.g., several DI waterbaths);

placing the film onto a clean substrate and air drying it;

dissolving the PMMA with a solvent (e.g., acetone);

patterning the graphene into a ribbon with e-beam lithography andoxygen;

depositing silver by thermal evaporation in a defined central area ofthe graphene ribbon;

chemically converting the silver into a silver halide;

after the chemical conversion of the silver into the silver halide,coating the structure with Al₂O₃;

removing the Al₂O₃ on the contact area, and patterning nickel (Ni) ontothe contact area; and

cleaning of the PMMA.

Embodiment 14

The method of embodiment 13, wherein the step of coating Al₂O₃ utilizestrimethylaluminum as a precursor.

A greater understanding of the subject invention and of its manyadvantages may be had from the following examples, given by way ofillustration. The following examples are illustrative of some of themethods, applications, embodiments and variants of the subjectinvention. They are, of course, not to be considered as limiting theinvention. Numerous changes and modifications can be made with respectto the invention.

Example 1

To examine the stability of the device according to an embodiment of thesubject invention, an AgCl/graphene sample was stored in ambient air andillumination conditions for 6 months and subjected to a stress testcontinuously for 12 hours, i.e., a 12 hour temporal photo response testof AgCl/graphene (470 nm SiO₂) photodetectors with ˜54.9 μW white lightover the 22×16 μm² graphene channel area. As shown in FIG. 2(e), thephotocurrent exhibited no significant loss after 6 month storage andduring stress test. It is worth mentioning that the photo response ofthis graphene photodetector can be modulated with an external gate bias.As shown in FIG. 2(f), higher photo response is observed under negativegate voltage V_(g), which can be attributed to the longer lifetime oftrapped charges in an AgCl layer due to a higher Schottky barrier heightwith lower graphene Fermi level. In particular, FIG. 2(f) illustratesthe photocurrent dependence on back-gate voltage from −200 V to 200 V.

Example 2

AgCl/graphene on Transparent Substrates

In order to explore the wavelength-dependent photo response ofAgCl/graphene photodetector, the AgCl/graphene device, according to anembodiment of the subject invention, was fabricated on a quartzsubstrate and measured with a supercontinuum laser. FIG. 3(a) shows thephoton-to-current efficiency η, which results from the ratio between thenumbers of electrons over the Schottky barrier multiplied by the gainvalue to the number of incident photons. The wavelength dependence of ηcan be explained by the Fowler theory, which describes the photon energydependent probability of photoelectrons being injected over the Schottkybarrier at the semiconductor-metal interface, resembling theAgCl-graphene interface in the subject invention. Knight providessupport for this proposition. In Fowler theory, the energy-dependentphotoemission probability

${\eta_{i} \approx {C_{F}\frac{\left( {{hv} - {q\;\Phi_{B}}} \right)^{z}}{hv}}},$where C_(F) is the device-specific Fowler emission coefficient, hv isthe photon energy, and qΦ_(B) is the Schottky barrier energy. Theincrease of η with decreasing wavelength can be understood byconsidering the higher η_(i) with increasing photon energy. Thedeviation of η from the quadratic increase of η_(i) in Fowler theory maypartially benefit from the hot carrier multiplication by virtue of theslow acoustic phonon cooling time in graphene (˜10 ns) (see, forexample, Sun.) A flexible visible photodetector can be fabricated onpolyethylene terephthalate (PET) film. See FIG. 3(b), which is an imageof a flexible AgCl/graphene photodetector on PET film being held betweena person's fingers. Due to low light absorbance of graphene and theultrathin AgCl layer, high transmittance ˜92.8% was recorded undernatural direct sun light illumination condition. The area ofAgCl/graphene is indicated in FIG. 3(b) by a dashed line.

Example 3

Wavelength-Resolved Photo Response on Thermal Oxide

It is well known that single layer graphene is visible on a siliconwafer with appropriate thermal oxide due to light interference insideand underneath the dielectric layers. In an embodiment of the subjectinvention, graphene itself is responsible for light absorption; thespectral response of a graphene photodetector can be selectivelyenhanced by modulating the thickness underneath the dielectric layer,which creates a color-sensitive photodetector. The wavelength-dependentresponsivity of the AgCl/graphene photodetector on 200 nm and 470 nmthermal oxide are shown respectively in FIG. 4(a) at a 1V bias (circlesfor 470 nm SiO₂ and squares for 200 nm SiO₂, left axis) andFinite-difference Time-domain (“FDTD”) simulated absorption spectra(solid line for 470 nm SiO₂ and dashed line for 200 nm SiO₂, rightaxis). All the data are normalized by the value at 400 nm wavelengthindividually. Different dielectric layer thicknesses lead todramatically different wavelength responses. The experimental photoresponse peaks match well with the absorption peaks obtained by the FDTDsimulation. In contrast to the response spectrum of AgCl/graphene, thephoto-response peak at 400 nm in AgBr/graphene and the AgI/graphenespectrum will merge with the band absorption of AgBr (E_(g)=2.69 eV) andAgI (E_(g)=2.83 eV) (see, for example, FIG. 6). FIG. 6 is a graph ofresponsivity versus wavelength for AgL/graphene (circle) andAgBr/graphene (inverted triangle). This in turn confirms that theabsorption at 400 nm and beyond are due to graphene absorption ratherthan the band gap absorption of AgCl (E_(g)=3.25 eV) (see, for example,Bose.) In particular, FIGS. 5(a)-5(c) are a FDTD simulation for awavelength selective enhancement where FIG. 5(a) is a normalizedintensity map of an absorption cross section for AgCl/graphene as afunction of both the excitation wavelength and the SiO₂ thickness, FIG.5(b) is a graph comparing the absorption cross section spectra ofAgCl/graphene with pure graphene on a 470 nm thick SiO₂ substrate, andFIG. 5(c) shows the power absorption per unit area plot for anAgCl/graphene device on 470 nm SiO₂ at 400 nm wavelength.

It is expected that other wavelength-selective enhancement techniques,such as photonic structure, waveguide, micro-cavity, and metal plasmonicenhancement, can also be applied to an embodiment of the subjectinvention, which promises great flexibility for designingcolor-sensitive photodetectors.

The responsivity of AgCl/graphene on 470 nm SiO₂ at 400 nm and 500 nmwavelengths, respectively, as a function of applied bias V_(sd) areshown in FIG. 4(b) and FIG. 4(c), respectively. A high peak responsivityof ˜880 A·W⁻¹ and ˜620 A·W⁻¹ can be obtained with 1.46 nW (400 nmincident light) and 0.94 nW (500 nm incident light) optical powerrespectively on the channel area (22×16 μm²) and 1.5 V. The value forV_(sd) is around 10 times higher than that of a commercial Si-avalanchephotodiode and could be further improved with higher V_(sd). The highphoto responsivity in the photodetector in an embodiment of the subjectinvention maybe due to the high photoconductive gain created byrecirculating of holes in the graphene channel within the lifetime ofthe photo-generated electrons stored in the Ag⁺/Ag⁰ reservoir. It isnotable that a higher responsivity is obtained under lower lightintensity because the filling of the Ag⁺/Ag⁰ reservoir will decrease theprobability of capturing photo-generated electrons. Thus, the excessivephoto-generated electrons will recombine with the holes in the graphenechannel quickly and shorten the average τ_(lifetime) and diminish thegain and responsivity value, as observed in ZnO-graphene system (see,for example, Shao).

Example 4

In order to examine the Ag metal plasmonic contribution to the device,phototransistors using silver halide-graphene hybrid (AgX-G, X═Cl, Br,I), as depicted in the schematic diagram FIG. 7(a), were fabricated. Thedegenerately p-doped Si substrate with 470 nm thermal oxide was used asback-gate electrode to modulate the Fermi level of the AgX-G hybrid. Thewell-dispersed silver nanoparticles were deposited on the CVD grapheneby thermal evaporation. Then the Ag nanoparticles were completelyconverted into AgX nanoparticles by allowing the Ag nanoparticles toreact with respective halogen gas (2Ag_((s))+X_(2(g))→2AgX_((s))) asobserved in X-ray photoelectron spectroscopy. After reaction, the AgXnanoparticles are well-dispersed on the graphene surface with averagediameter ˜18 nm. Finally, 5 nm Al₂O₃ was coated over the device surfacewith atomic layer deposition (ALD) to minimize the influence of ambientgas molecule adsorption and desorption upon illumination. In order toestimate the possible contribution of the plasmonic absorption from thetrace amount of unreacted Ag metal on the graphene surface, a controldevice was fabricated with silver nanoparticles over graphene surface.As no detectable photoresponse was observed in this control sample, itis safe to conclude that the Ag metal plasmonic contribution to thephotoresponse in an embodiment of the subject invention isinsignificant.

Example 5

The photoresponse of hybrid phototransistors with different silverhalides, (AgCl, AgBr or AgI) under same visible illumination wasexamined. Since the AgCl is a wide bandgap (E_(g) ^(AgCl)=3.25 eV)semiconductor, only ultraviolet sensitivity would be expected, assumingthe same sensing mechanism as previously reported forgraphene-semiconductor hybrid phototransistor. In contrast, a highphotoresponse is observed with visible light illumination, suggesting anew sensing mechanism in our phototransistor. FIG. 7(b) shows thephotocurrent (after dark current subtraction) of the as-prepared AgX-Gphotodetectors (e.g., phototransistors) at room temperature underchopped light illumination (white light illumination with power of 54.9nW) at 1 V source-drain voltage (V_(SD)) and 0V back-gate voltage(V_(G)). If working as traditional graphene hybrid phototransistor wherethe semiconductor is responsible for light absorption, the photoresponseof AgI-G hybrid should be highest as the AgI has the lowest bandgap andthe highest photosensitivity. However, the observed photoresponse ofAgCl-G hybrid is much stronger than that of AgBr-G and AgI-G, whichstrongly suggesting that the photoresponse is not originated from AgXnanoparticle absorption. This phenomenon can be attributed to thesub-bandgap photosensitivity, in an embodiment of the subject invention,to the electrochemical assisted internal photoemission from graphene toAgX nanoparticles. As illustrated in FIG. 7(c), upon illumination withlow energy photon, the photoexcited hot carriers with energy higher thanthe Schottky barrier height (Φ_(B)) can be injected into AgXnanoparticles and then stored into the low-energy redox level of AgX/Ag⁰via inducing the half electrochemical reduction reaction AgX+e⁻→Ag⁰+X⁻,which will negatively charge the AgX nanoparticles and enhance theconductance of graphene transistor through capacitive coupling. In thedark, the reduction reaction is reversed and the injected electron isreleased back to graphene through tunneling process, restoring thephototransistor back to its low conductance state. Here, the sub-bandgapphotoresponsivity of the graphene hybrid detector is primarilydetermined by the nanoparticle's ability of accepting electrons. Inelectrochemistry, the electrochemical potential (E) is classically usedto represent the ability of accepting electrons for an oxidant. In ourcase, since the AgCl is a stronger oxidant than AgBr and AgI(E_(AgCl/Ag)=−4.66 eV<E_(AgBr/Ag)=−4.51 eV<E_(AgI/Ag)=−4.29 eV) andreadily retains the injected hot carriers from the graphene, the highestgain and photoresponsivity in AgCl-G followed by AgBr-G and AgI-G isobserved. This result suggested the classical standard electrochemicalpotential could also be used to benchmark the electron accepting abilityof other electrochemical reaction assisted graphene hybridphototransistors for low energy photon sensing. Without favorable redoxcouple (e.g. in ZnO-graphene hybrid), the internal photoemission fromgraphene to semiconductor is insignificant and the sub-bandgapphotoresponse is negligible. Since the phototransistor according to anembodiment of the subject invention involves electrochemical redoxreaction, it is important to assess the stability of our device, whichis determined by the reversibility of the redox reaction. The AgCl-Gsample was stored under ambient conditions for 6 months and thensubjected to a long time test under chopped illumination, as seen inFIG. 7(d). After over 1000 on-off cycles, the photocurrent maintainedthe same level, indicating the great reversibility and reliability ofour device.

Example 6

To confirm the sensing mechanism in the phototransistor, aphotoelectrochemical electrode based on AgCl-G hybrid was fabricated totest the internal photoemission current from graphene to AgClnanoparticles at 0 V bias. As shown in FIG. 7(e), AgCl nanoparticlesloaded monolayer graphene was used as a working electrode and submergedinto 1 M KCl solution with AgCl/Ag as the counter electrode. Uponillumination with low energy photons (520 nm), the AgCl cannot beexcited while the hot electron is generated in graphene. The temporalphotocurrent at 0 V bias in chopped green light illumination wasrecorded, as shown in FIG. 7(f). Since no detectable photocurrent isobserved for AgCl nanoparticles over platinum surface which has similarwork function as graphene (Φ_(graphene)=4.9˜5.1, Φ_(Pt)=5.1), thephotocurrent is clearly not originated from the AgCl absorption andshould be attributed to the graphene internal photoemission, similar tothe recently demonstrated graphene-semiconductor photodetector. In thismechanism, since the internal photoemission is controlled by theSchottky barrier height at the graphene-semiconductor interface insteadof the bandgap of incorporated semiconductor, our photodetector iscapable of exploiting sub-bandgap photons and extending the sensitivityto the visible spectrum. This strategy offers a new approach to engineerthe spectral response of optoelectronic devices via surface engineeringwithout using narrow bandgap materials.

Example 7

To demonstrate that, in a phototransistor, the spectral selectivity canbe accomplished by simply modulating the thickness of thermal oxideunderneath graphene layer, as seen in FIG. 8(a). As shown in FIG. 8(b),for AgCl-G on a silicon substrate with 470 nm SiO₂, two majorresponsivity peaks, located at 400 nm and 500 nm are recorded. Thisenhancement can be attributed to the optical resonance inside thedielectric layer. The power absorption spectrum predicted by thefinite-difference-time-domain (FDTD) simulation matches well with theexperimental data. As a comparison, only a single photoresponse peaklocated around 400 nm can be identified for the photodetector fabricatedon 200 nm SiO₂ substrate as calculated by the FDTD. An overview forspectral selectivity by changing the oxide thickness is shown in FIG.8(c).

In contrary to a previously demonstrated graphene-semiconductorphototransistor, an embodiment of the subject invention relies on thephoton absorption in graphene layer rather than in semiconductor. Asshown in FIG. 8(d), the simulated absorption spectra of AgCl-G and baregraphene match well with each other in the visible region, indicatingthat the light absorption in AgCl is negligible for AgCl-Gheterostructure. The power absorption ratio between graphene(P_(graphene)) and AgCl (P_(AgCl)) is 126:1 for 400 nm incidentwavelength and even higher for 500 nm (246:1), which again confirms thedistinct sensing mechanism of AgCl-G hybrid phototransistor.

It should be understood that the examples and embodiments describedherein are for illustrative purposes only and that various modificationsor changes in light thereof will be suggested to persons skilled in theart and are to be included within the spirit and purview of thisapplication.

All patents, patent applications, provisional applications, andpublications referred to or cited herein (including those in the“References” section) are incorporated by reference in their entirety,including all figures and tables, to the extent they are notinconsistent with the explicit teachings of this specification.

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What is claimed is:
 1. A photodetector comprising: a supporting medium;a layer of graphene on the supporting medium; and a layer ofsemiconductor nanoparticles in direct, physical contact with the layerof graphene, wherein the semiconductor nanoparticles are semiconductorswith bandgaps larger than the energy of photons intended to be detectedby the photodetector, and wherein the semiconductor nanoparticles aresilver halide nanoparticles.
 2. The photodetector of claim 1, whereinthe layer of semiconductor nanoparticles is on the layer of graphene. 3.The photodetector of claim 1, wherein the layer of semiconductornanoparticles is under the layer of graphene.
 4. The photodetector ofclaim 1, wherein the layer of semiconductor nanoparticles is mixed inwith the layer of graphene.
 5. The photodetector of claim 1, wherein thegraphene is chemical vapor deposition grown, liquid exfoliated, ormechanically exfoliated.
 6. The photodetector of claim 1, wherein thediameter of the semiconductor nanoparticles is from the nanometer rangeto the micrometer range.
 7. The photodetector of claim 1, wherein thesemiconductor nanoparticles are AgCl nanoparticles, with Eg=3.25 eV,which detect photons with energy of less than 3.25 eV.
 8. Thephotodetector of claim 1, wherein the semiconductor nanoparticles aresensitized by organic or inorganic dyes so as to modulate theirproperties.
 9. The photodetector of claim 1, wherein the supportingmedium is one or more of quartz, thermal oxidized Si, sapphire, siliconcarbide, aluminum nitride, polydimethylsiloxane, and a flexible plasticsubstrate.
 10. The photodetector of claim 8, wherein the supportingmedium is integrated with a photonic structure to modulate theperformance of the device, wherein the photonic structure is a microcavity, a waveguide, or a metal plasmonic structure.
 11. Thephotodetector of claim 1, wherein the thickness of the supporting mediumis modulated to achieve selective enhancement of photodetection.
 12. Amethod of performing flexible and transparent optoelectronics forimaging, spectroscopy, sensing, or optical communications, the methodcomprising: providing a photodetector according to claim 1; and usingthe photodetector to detect photons having an energy smaller than thebandgaps of the semiconductor nanoparticles.